tox: Remove ineffective ignore_basepython_conflict and bump minimum version

Change-Id: I46d7512ffb1788d1d16023aac2945df3ed5b2f26
Signed-off-by: Ivan Anfimov <lazekteam@gmail.com>

tox.ini

@@ -1,11 +1,9 @@
 [tox]
-minversion = 3.1
+minversion = 4.6.0
 skipsdist = True
 envlist = docs,linters,functional
-ignore_basepython_conflict = True
 
 [testenv]
-basepython = python3
 usedevelop = False
 install_command =
     pip install -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master} {opts} {packages}
@@ -33,7 +31,9 @@ setenv =
     WORKING_DIR={toxinidir}
 
 [testenv:docs]
-deps = -r{toxinidir}/doc/requirements.txt
+deps =
+  -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+  -r{toxinidir}/doc/requirements.txt
 commands =
     bash -c "rm -rf doc/build"
     doc8 doc
@@ -52,7 +52,7 @@ commands =
 extensions = .rst
 
 [testenv:releasenotes]
-deps = -r{toxinidir}/doc/requirements.txt
+deps = {[testenv:docs]deps}
 commands =
     sphinx-build -a -E -W -d releasenotes/build/doctrees --keep-going -b html releasenotes/source releasenotes/build/html